In the alkaline-catalyzed polymerization of 2-pyrrolidone, alkali metal hydroxides, particularly sodium and potassium hydroxides, have commonly been used as sources of alkaline catalyst. The reaction of the hydroxide with 2-pyrrolidone produces the alkali metal pyrrolidonate catalyst, and as a by-product, water. The reaction is believed to be represented by, e.g., for KOH, the following equation: ##STR1## The reaction has been typically carried out in excess 2-pyrrolidone from which the water of reaction is then removed leaving a substantially anhydrous solution of alkali metal pyrrolidonate catalyst in 2-pyrrolidone. The anhydrous solution is then subjected to polymerization conditions. This may be termed the in situ method of catalyst preparation and polymerization.
Elevated temperatures can be used to effect dissolution of the hydroxide in 2-pyrrolidone and subsequent removal of water. But these conditions lend themselves to the hydrolysis of 2-pyrrolidone and subsequent interference by the hydrolysis products with polymerization (see U.S. Pat. No. 3,778,402). Consequently, the art teaches one to dissolve solid hydroxide, typically in the form of KOH pellets (85% KOH), in an excess of 2-pyrrolidone and to remove water under conditions of reduced pressure (see U.S. Pat. No. 3,721,652). The polymerization yield of polypyrrolidone and the polypyrrolidone molecular weight are extremely sensitive to monomer treatment. Hydrolysis of the sensitive 2-pyrrolidone ring, which is said to occur even under mild conditions of prolonged heating of aqueous 2-pyrrolidone, produces polymerization inhibitors which can depress the yield of polypyrrolidone (see U.S. Pat. No. 3,778,402). On the other hand, dehydration of 2-pyrrolidone, which is said to occur even under mild conditions of prolonged heating of anhydrous 2-pyrrolidone, produces polymerization activators which can cause "runaway polymerization" and the production of low-molecular-weight polypyrrolidone (see U.S. Pat. No. 3,184,476).